iArchiver is an archive tool that supports many
formats that Apple forgot. It creates DMG, 7-zip,
Zip, gzip, bzip2, and many other file formats in
three easy steps. It can also convert between file
formats.

SCSU is a Standard Compression Scheme for Unicode, implemented for PHP and JavaScript. It's suitable for better LZF compression for UTF-8 strings and is based on the Java source of SCSU by Unicode, Inc. Its PHP classes are self-documented by PHPDoc tags, and its JavaScript class by JSDoc tags.

iZip provides an easy way to manage .ZIP, .ZIPX, and .RAR archives on Mac systems. When you open an archive with iZip, it appears as a removable disk. This allows management of your files right inside the OS X Finder. All view modes (including cover-flow) just work. For extra security, iZip supports up to 256 bit AES zip encryption.

gqviewarc is an LD_PRELOAD module that makes
GQview treat archives (ZIP, RAR, and maybe more)
as directories. It may also be applicable in other
domains where transparent stream read access to
archived files is needed.

Zipeg is free utility that opens and explores the
contents of zip, rar, arj, and other archives. It
can preview files and images before you extract
them, and lets you extract only the files you
want. Many documents and pictures can be emailed
as zip archives.

crunch/crnlib is a lossy texture compression tool and library for developers that distributes and uses content in the DXT1/5/N or 3DC/BC5 compressed mipmapped texture formats. crnlib can compress mipmapped 2D textures and cubemaps to .8-1.25 bits/texel, and normal maps to 1.75-2 bits/texel. crnlib's quality and performance is competitive to transform-based solutions, or other offline/real-time DXTn compressors such as squish or ATI_Compress. crnlib implements a new form of "clustered" DXTn compression, with a compressed texture data format that was carefully designed to be very quickly transcodable directly to raw DXTn texture bits with no intermediate recompression step or individual pixel-level operations. The typical single threaded transcode to DXTn rate is equivalent to 100-250 megatexels/sec. Fast random access to individual mipmap levels is supported.

saltyfiles is a platform independent encryption tool with a
simple user interface. It uses 'Password Based
Encryption' (PBE) to encrypt and/or decrypt files on the
filesystem. Encrypted files will also be compressed using the
zip format.

Pipa reads data from stdin and does various things with it: rotates by time interval / output file size; compresses it (gz, bz2, lzo, xz); names the output file with strftime(); splits it intelligently when given info about the file structure (newlines / PCAP); calls a user command on the finished output file; passes it to stdout (like tee); and more.